In general, a 24 V power supply is used in a diesel engine having large engine displacement, and a 24 V power supply is used in a working machine such as a relatively large-sized hydraulic excavator for both an electrical system of a machine body and an electrical system of an engine.
However, an engine having smaller engine displacement than that of a conventional model can be used in a relatively small-sized hydraulic excavator and the like, and an engine of this class mainly uses a 12 V power supply for the electrical system, which is different from a 24 V power supply for the electrical system of the machine body. To change the electrical system of the engine to 24 V to match the electrical system of the machine body, or to change the electrical system of the machine body to 12 V to match the electrical system of the engine requires much development cost and a long development period. Therefore, it is desirable to use power circuits of 12 V and 24 V by using two 12 V batteries (refer to Patent Literature 1 and 2, for example).
Further, as shown in FIG. 6, a power supply device for vehicles that has a power supply circuit having a low voltage load circuit 3 and a high voltage load circuit 4 connected to a plurality of batteries 1 and 2 connected in series uses a technique that an equalizer (battery balancer) 6 is connected to a ground 5 and to circuits 3a and 4a that output a low voltage and a high voltage from the plurality of batteries 1 and 2 to the low voltage load circuit 3 and the high voltage load circuit 4 so that the equalizer 6 prevents unsymmetrical wear of the batteries 1 and 2.
Further, there is a technique that, in a ground circuit 5a that connects the ground 5 and a minus terminal (−) of one of the batteries 1 and 2 connected in series, which is connected to the ground 5, there is provided a circuit breaking switch (hereinafter, a disconnect switch) 7 that can break the circuit 5a so that the disconnect switch 7 mechanically breaks the ground circuit 5a to prevent battery exhaustion at a vehicle maintenance work time or during a long-term storage. A stator motor 8 and an alternator 9 of the vehicle are connected in parallel to the high voltage load circuit 4.
Patent Literature 1: Japanese Patent Application Publication No. 2010-093979
Patent Literature 2: Japanese Patent Application Publication No. 2013-135489